This invention relates generally to systems for recovering information from recording discs, and, more particularly, to a system that moves an information recovery device rapidly toward a selected target track on a recording disc to recover information recorded on it.
An example of a system of this particular type is described in a copending application for U.S. patent Ser. No. 295,629, filed Aug. 24, 1981 and entitled "Method and Apparatus for Information Retrieval from an Optically Readable Storage Medium," and assigned to the assignee of the present application. The disclosed apparatus directs a reading beam of light at a selected one of a plurality of substantially circular and concentric recording tracks on a rotatable recording disc. The intensity of the beam is modulated by the recorded information, which includes a unique address signal for each track and the apparatus detects the modulated beam to produce a playback signal indicative of the recorded information.
The apparatus includes means for selecting a particular target track to be scanned, along with a carriage motor and a movable mirror for moving the reading beam radially toward the target track. The apparatus further includes means for monitoring the playback signal to detect periodically the address of the track currently being scanned. This current address is compared to the target address, and the apparatus applies a prescribed sequence of drive signals to the carriage motor, depending on the remaining distance to the target track. The speed of the carriage motor is stepped successively downwardly as the reading beam reaches predetermined distance thresholds during carriage translation. During the last stage of carriage translation, the movable mirror increments the beam radially by one track spacing during each revolution of the disc, thereby "playing" into the target track.
Although the information recovery system described above has proven effective in rapidly moving a reading beam toward a selected target track, to recover information recorded on it, the system is not believed to operate as rapidly as is possible. The final stage of carriage translation, in which the beam plays into the target track, is believed to be particularly time consuming. There is therefore still a need for a system that can move an information recovery device even more rapidly toward a selected target track, to recover information recorded on it.
While the carriage motor moves the reading beam radially in the direction of the target track, the system disclosed in the aforementioned application periodically updates a storage register indicating the address of the particular track currently being scanned. In one disclosed embodiment, this updating is achieved by detecting successive track crossings and incrementing (or decrementing) the stored address with each crossing. Although this technique is generally effective when traversing tracks at relatively high rates, it has not performed well when traversing at relatively low rates because eccentricities in the recording disc can actually cause the beam to move relative to the disc in a radial direction opposite to that of the carriage motor.
A second disclosed embodiment of the aforementioned application updates the storage register each time it successfully detects an address in the playback signal. This technique is effective whenever the beam traverses tracks at a relatively low rate, but generally is not completely effective when the beam traverses at relatively high rates. This is because the reading beam frequently does not scan a single track for a long enough duration to recover the entire address recorded on it.
Whenever the system described above does not have an accurate indication of the track currently being scanned, and thus of the remaining radial distance to the target track, the system will likely not drive the reading beam toward the target track in an optimum fashion. It should therefore be appreciated that there is a need for a system that provides a more accurate indication of the current location of the information recovery device in relationship to the target track, such that the device can be moved as rapidly as possible in the direction of the target track, without overshooting it. The present invention fulfills this need.